By combining the ability of accelerator mass spectrometry to detect low levels of 14C with the high sensitivity of high energy collision-induced dissociation analysis for peptide sequence determination, we have developed a novel approach to detect and characterize proteins with adducted xenobiotic compounds. In the present study, we are trying to identify benzene-protein adducts purified from mouse liver and bone marrow cells. Benzene is a commonly occurring toxic compound and is present in combustion products, gasoline and tobacco smoke. It is known that chronic exposure to benzene leads to degeneration of the bone marrow and leukemia. Further, it has also been shown that benzene-protein adducts are formed via metabolism of benzene. Bioactivation of benzene occurs in the liver through oxidation by P450 to phenol. However, several other reactive metabolites are formed as well, such as benzoquinones, catechol hydroquinone and muconaldehyde. The hematotoxic effect of benzene has been attributed to these benzene intermediates rather than benzene or phenol. The aim of this work is to characterize the protein adducts formed with benzene and to elucidate their possible importance for the development of leukemia and other benzeneinduced pathological conditions. Crude liver homogenates obtained from mice injected with low amounts (500 ng/kg body weight) of 14C-benzene are fractionated by SDS-PAGE and the 14C-enriched proteins are detected by accelerator MS. The proteins of interest are subjected to in-gel trypsin digestion and the extracted peptides sequenced by high energy CID analysis.